``The decrease in body size was due to a reduction in growth rates caused, in part, by the changing climate,'' Coulson said in an interview via e-mail.

Evolution favors the development of large sheep, which can more easily survive harsh winters, Coulson explained. So the researchers became curious about the overall decline in size of the animals on Hirta.

They discovered that as the climate has grown milder, small lambs that would not have survived previous winters were now living to grow up and reproduce.

Since size is inherited, the survival and reproduction of these smaller animals lowered the average size of the herd.

In addition, Coulson noted, there is what he termed the “young mum effect,'' with the younger mothers physically unable to produce large offspring.

The find adds to the understanding of how change occurs in many types of animals, he said, including birds, fish and mammals.

It shows how evolution and ecology each play a role in change, Coulson said: “And that, for our wild sheep at least, climate change is having a detectable effect on body size, a trait that is partly determined by genes, and that this compliments previous research showing how climate change can influence population size.''

“This study addresses one of the major goals of population biology, namely to untangle the ways in which evolutionary and environmental changes influence a species' traits,'' said Andrew Sugden, deputy and international managing editor at Science, which published the report. The research was supported by Britain's Natural Environment Research Council.